The present invention relates to a coated tool for warm-and/or-hot working such as a forging die etc. used in circumstances where metals are in sliding contact with each other in a warm or hot condition.
As for a conventional tool for the warm-and/or-hot working, a hot working die steel defined in JIS-SKD61 or JIS-SKT4 is usually used, and in a case where the durability of the die is particularly required, there is used a steel, which has a high-temperature strength higher than that of JIS-SKD61 or JIS-SKT4, such as JIS-SKD7, JIS-SKD8, high speed steel or a steel obtained by improving each of them.
For example, in order to satisfy requirements for a warm-and/or-hot forging die (hereinafter, referred to as xe2x80x9cdiexe2x80x9d) such as the enhancement of the working efficiency, the high precision design of a worked product and a near-net-shape design, a nitriding such as a plasma process, a salt bath process or a gas process etc. comes to be used, or a physical vapor deposition coating such as an arc discharging ion plating method etc. (hereinafter, referred to as xe2x80x9cPVDxe2x80x9d) in combination with the nitriding comes to be used, for the purposes of enhancing the wear resistance of the working surface of the die, the anti-seizure property thereof and the heat crack resistance thereof while keeping the toughness of the die.
In JP-A-11-92909, in order to enhance the adhesion strength between a surface of die material and a PVD coating, there are disclosed the regulating of the surface roughness of the surface of the die material to be coated which regulating is performed by use of a diamond paste, the applying of a vacuum gas nitriding, and a washing by a electrolytic process as a pre-treatment to be performed before coating such as CrN or TiAlN which is formed by PVD. Further, JP-A-11-152583 discloses the using of both of a nitriding and the PVD coating of TiN, CrN or TiCrN for the purpose of improving the heat crack resistance and oxidization resistance property of the die surface.
However, each of JP-A-11-92909 and JP-A-11-152583 brings about only the improvement of service life of 20 to 30 percents in comparison with the conventional dies, that is, in these prior arts it is impossible to achieve a drastic improvement of the service life of the die, and these prior arts are not necessarily satisfied insofar as the requirements such as the improvement of the working efficiency, the high precision design of the worked product and the near-net-shape design are concerned.
Particularly, since the near-net-shape design of the worked product causes the shape of a product to become complex, not only the rate of a metal flow of the worked material increases, but also a stress applied to the working surface of the die increases during the working, so that seizure, galling and so on come to occur between the working surface of the die and a worked material at an early stage of an operating. One of the factors causing this problem is thought to be the breaking-off of a lubricant occurring by the severity of the forging condition.
The occurrence of the seizure, galling and so on come to cause excessive friction forces at the interface defined between the working surface of the die and the worked material, so that much friction-heat occurs. As the result thereof, the material is extremely softened at a surface portion of the die due to the heat, and the coating on the die surface is easily peeled off so that the wear resistance of the die is extremely deteriorated. Depending on the product shape, there occurs such a case as the temperature of the die becomes a high temperature exceeding the transformation point of the die material itself (700xcx9c900xc2x0 C.), that is, a circumstance to which the die is exposed becomes very severe.
In addition, the complication of the product shape due to the near-net-shape design of the worked product causes a great variation of the metal flow rate of the worked material in dependence on the location of the die. In other words, at an early stage of the forging in which the surface temperature of the die is unstable, the surface temperature of the die raised due to the friction-heat occurring between the worked material and the die comes to be greatly varied in dependence on the location thereof.
Regarding the warm-and/or-hot forging, the present inventors have found the phenomenon and problems explained below. In the warm-and/or-hot forging, although a lubricant is applied by spraying after every each forging operation, the lubricant has such a characteristic as the largest amount of adhered lubricant at a particular die surface temperature. That is, the fact that the surface temperature greatly varies in dependence on the location thereof means that the amount of the lubricant present on the die also varies greatly in dependence on the location thereof, and there occurs such an unfavorable state as a suitable amount of lubricant is present regarding certain positions and as a lubricant-shortage or no lubricant occurs at other positions. Of course, at the position where the amount of the adhered lubricant is low, seizure galling and so on between the die and the worked material is apt to occur at an early stage of the forging.
Nowadays, the PVD coating used for the warm-and/or-hot forging die is improved mainly regarding the adhesion strength between the die surface and the coating, so that there occurs such a problem as the premature seizure and/or galling are caused when the die having the coating is used in the circumstance in which the amount of the lubricant varies as explained above or in which the friction heat is caused very much, so that the coating is peeled off without sufficiently exerting its effect.
An object of the present invention is to provide a coated tool for warm-and/or-hot working with superior anti-seizure property and wear resistance, which tool can solve the above problems.
The inventors of the invention have investigated in detail regarding the influences of each of the composition of the PVD coating, the layered structure of the coating and the coating on each of the adhesion of the lubricant applied onto the warm-and/or-hot working tool and the anti-seizure property thereof.
As the result thereof, the inventors have found that, by coating on the die surface of a warm-and/or-hot working tool a layer of at least one of nitride, carbide and carbonitride which layer contains as the main constituent thereof at least one metal element selected from the group consisting of Ti, V, Cr, Al and Si, and by coating on this layer another layer of a sulfide, it is possible to obtain a superior anti-seizure property and a superior wear resistance both required in the tool for warm-and/or-hot working. According to this, it is confirmed that, in a case of, for example, a hot forging die provided with this layered structure, it becomes possible to sufficiently suppress a local seizure at an initial stage of the forging and to suppress excessive heating occurring due to the sliding contact between a worked material and the die after a middle stage of the forging, and that the service life of the hot-forging die is remarkably enhanced.
That is, according to the first aspect of the invention, there is provided a coated tool for warm-and/or-hot working with a superior anti-seizure property and a superior wear resistance, comprising: a base material selected from the group consisting of a hot die steel and a high speed steel; and a coating as a working surface, the coating having: a layer xe2x80x9caxe2x80x9d provided on the base material, the layer xe2x80x9caxe2x80x9d being made of at least one material selected from the group consisting of a nitride, a carbide and a carbonitride, each of which contains as the main constituent thereof at least one metal element selected from the group consisting of Ti, V, Cr, Al and Si; and a layer xe2x80x9cbxe2x80x9d provided on the layer xe2x80x9caxe2x80x9d,the layer xe2x80x9cbxe2x80x9d being made of a sulfide.
The layer xe2x80x9cbxe2x80x9d of the sulfide preferably contains, by atomic % in terms of only metal compositions, at least one not more than 50% (including 0%) in total selected from the group consisting of Ti and Cr and the balance substantially Mo, and the layer xe2x80x9cbxe2x80x9d preferably has a thickness of 0.5xcx9c10 xcexcm.
In the invention, the superior anti-seizure property and superior wear resistance can be obtained by, for example, providing the sulfide layer xe2x80x9cbxe2x80x9d as the outermost layer of the coating. Further, by regulating the surface roughness of the outermost layer to a predetermined value, the amount of the lubricant present on the surface can be increased, which is particularly effective at the early stage in which the surface temperature of the tool is unstable.
Namely, it is preferable that, as the outermost layer of the coating, there exists a layer xe2x80x9ccxe2x80x9d having a surface roughness of Rz: 4xcx9c15 xcexcm which layer xe2x80x9ccxe2x80x9d contains as the main constituent thereof at least one metal element selected from the group consisting of Ti, V, Cr, Al, Si and Cu, and it is preferred that the layer xe2x80x9ccxe2x80x9d has a thickness of 2xcx9c15 xcexcm.
Furthermore, the layers of the coating are preferably formed by the physical vapor deposition, and it is preferred that the coated substrate or material has at a depth of 25 xcexcm from the outermost surface of the coated substrate or material a hardness higher, by not less than 200 HV0.2, than that of a portion of 500 xcexcm depth, in which the xe2x80x9cHV0.2xe2x80x9d is a mark defined in JIS-Z-2244 which mark shows a Vickers hardness measured under a load of 1.961 N.